Electrochemical button cells are suitable for use in applications in which the space available for a battery is minimal, such as in hearing aids and other small electrically-powered devices. The phrase “button cell” generally refers to the shape of the cell being relatively disc-shaped, like a garment button. One common button cell having relatively high energy density is a metal-air cell, which comprises a metal-containing anode and an air cathode. However, the small size and limited amount of electrochemically reactive material in even a metal-air cell limits the useful operating life of the cell.
FIG. 1 is a cross section of a metal-air button cell, generally designated 1, comprised of an electrically conductive anode can 5 received in an electrically conductive cathode can 10. An air cathode assembly 14 overlays the bottom wall 12 of the cathode can 10 within the cell and is in electrical contact with the cathode can. Anode material 6, which is a mixture of metal powder (commonly zinc) and an electrolyte (e.g., a solution of potassium hydroxide and water) is contained within the anode can in electrical contact therewith. A separator 18 separates the air cathode assembly 14 from the anode material 6.
A generally cylindrical, and more particularly annular, thin-walled dielectric (e.g., electrically non-conductive) grommet 25 (also commonly referred to as a “gasket” or “seal”) electrically insulates the anode can 5 from direct electrical contact with the cathode can 10 and forms a seal therebetween to sealingly close the reactive materials within the cell 1. The grommet 25 has an annular or tubular sidewall 29 having a thickness that is essentially the same, top to bottom, with an integrally formed annular foot 27 extending radially inward from the lower end or bottom of the sidewall. The foot 27 defines an annular shoulder 31 on which the terminal (e.g., lower) end 33 of the anode can sits upon assembly of the cell 1. The terminal (e.g., upper) end 39 of the cathode can 10 is crimped over the edge margin of the top of the anode can 5 to secure the cell 1 in its assembled configuration.
Metal-air cells such as that illustrated in FIG. 1 take in atmospheric oxygen via openings 19 formed in the bottom of the cathode can 10, and convert the oxygen to hydroxyl ions in the air cathode assembly 14 by interaction with the electrolyte, when the cell is being discharged. The hydroxyl ions then migrate to the anode material 6, where they interact with the metal anode material, which undergoes an oxidation reaction forming, for example, zinc oxide. Since the overall capacity (e.g., useful life) of any electrochemical cell is to some extent determined by the quantity of electrochemically reactive materials (e.g., the volume of anode materials) within the interior of the cell 1, and since a metal-air cell stores only the anode material internally (using atmospheric oxygen as the other reaction component), a common goal in cell design is to maximize the size (e.g., volume) of the interior cavity, and in the case of metal-air cells, more specifically the anode cavity.
It is understood that by making the grommet 25 thinner, the volume of the interior cavity of the cell could be increased (e.g., by allowing the inner diameter of the anode can to increase) without increasing the outer size of the cell. Alternatively, the size of the cell could be made smaller without decreasing the useful life of the cell if the grommet were made thinner. However, thinner grommets may present issues during fabrication of the cells and may also present cosmetic issues in finished cells. For example, thinner grommets may lose their circular shape or may collapse entirely during fabrication of cells. In addition, the portion of thinner grommets situated near the point where the terminal end 39 of the cathode can 10 is crimped over the edge margin of the top of the anode may wrinkle, negatively impacting the appearance and/or seal of the finished cell. Thus, the need exists for a grommet that provides increased volume of the interior cavity of the cell, or that allows for reducing the size of the cell without decreasing the useful life of the cell, but that also exhibits sufficient structural integrity to avoid the problems that may be associated with relatively thin grommets during cell fabrication and the cosmetic issues that may be associated with thin grommets in finished cells.